1. Technical Field of the Invention
This invention relates to a shock absorber and cushion for use during printing that compensates for variations in thickness, height and centricity of the materials and equipment used for printing to enhance the image quality and efficiency of the printing process without increasing printing pressure.
2. Description of Background Art
Flexography is a printing process used primarily in the packaging and newspaper industries. The flexographic and letterpress printing process requires that a raised surface plate be used to transfer ink onto a given substrate. This is unlike lithography, which uses a flat image carrier (plate) where non-print areas are aluminum and areas to print on are lacquer, based on the principle that oil and water do not mix. The gravure printing process is a recess process in which cells are engraved into the print cylinder that are then filled with ink and then transferred to the substrate.
Letterpress is similar to flexography utilizing a raised image carrier and also requiring a cushioning layer. Letterpress equipment is available as a platum or cylindrical plate mounting apparatus.
The flexographic printing process"" unique capabilities include changing cylinder dimension (circumference) to accommodate various length packaging. Printing presses in flexography can be as narrow as six inches or less and in excess of 120 inches wide. As in most manufacturing and machine processes, there is a plus or minus tolerance in gauge (thickness) uniformity which may include: the print cylinder uniformity, both across and around the web; a tolerance in the material surface being printing on; and, the tolerance of the back cylinder which the substrate rides on as it maneuvers through the press in addition to other mechanical elements. Variations in tolerances require excessive pressure during printing on the flexographic plate to overcome inaccuracies, which may smear and distort the print image such as halos and oval dots.
Currently the raised imaged carriers (flexographic plates) adhere to a print cylinder using various methods, which include clamps, pins, vacuum and most commonly, an adhesive tape applied to a flat seamless cylinder. There are various types of adhesive tapes used to adhere the flexographic printing plate to the cylinder. Although there are many variations of adhesive tape materials available, the materials used are routinely lumped into the following three categories:
(1) Hard Tapexe2x80x94no significant or claimed cushioning affect. This tape is best used when large amounts of ink need to be applied at 100% strength (full strength). However, since this tape has no inherent ability to even out the mechanical tolerances of the printing press, more than minimal pressure is normally required. This pressure creates a distorted printed image appearing in various forms that may include hard edges around the outer portion of the line copy while leaving a halo adjacent to this hard edge. Depending upon impression required, text may be squeezed to a point where it begins to slur (elongated the print in a through-press direction).
(2) Soft Tapexe2x80x94used as a cushion to allow even impression across and around the cylinder. This is because soft foam tape collapses or compresses under pressure in the areas that come into impression first which represent the largest circumference of the print package and must be impressed several thousands more until the entire image appears to be printing evenly and uniformly. Because of its softness, this material is used primarily when fine details or extremely small images are printed to help minimize the distortion that occurs under pressure with hard tape. Soft tape is traditionally used when printing half tones for screened pictorials, gradations and screen tints. Due to the soft nature of this cushioning element, the amount of pressure required to transfer a solid image is significantly compromised.
(3) Medium tape used as a cushioning element considered being of medium density. Medium tape is a compromise between the attributes of a soft tape used for printing fine graphics, and, hard tape used for images which need to print robust solids on the same printing surface using the same cushioning material.
The present invention eliminates or minimizes the negative attributes of the cushion product(s) described above available today. This includes inconsistency in gauge of the raw material currently available which is said to vary by plus or minus several thousands of an inch. With foam technology, foam cells or voids are filled with air, and during impression, under high spots, air is forced away and needs time to return to cells and thus return to initial tape height or dimension prior to the next revolution of the press. Cell inflation delay requires the press to run at lower speeds when working with a foam material, soft or medium. The slower drum speeds provide the time for the foam cell tape material to rebound between successive impressions. Throughout a very long print run, the foam material gradually loses ability to rebound. Constant monitoring is required throughout the run and most often results in color shifts and unacceptable print at some point in time, which is normally over one million impressionsxe2x80x94but in most cases not greater than three million impressions.
U.S. Pat. No. 3,285,799 discloses a printing blanket for long periods of use in offset lithography, which is composed of a polymeric film and woven backing, an ink transfer layer, and a resilient compressible support layer. The support layer has an external surface subdivided by grooves that leaves flat surfaced islands. The blanket is used as an intermediate to transfer an ink image from a printing plate to paper. The support layer has a durometer of at least 60 Shore A. The support layer contains at least about 0.005 cubic inches of voids per square inch of blanket surface but total void volume does not exceed 40%.
U.S. Pat. No. 5,325,776 discloses a cushioning backing sheet material positioned between a flexographic printing cylinder and a flexible printing plate. The cushioning sheet is an elastomeric material containing widely spaced, closed cell voids which provide pockets within which the encapsulated air can be pneumatically compressed when force is applied, and which all rebound rapidly when the force is relieved. A disadvantage of the closed-cell cushioning material fatigues and looses compression and resilience qualities, and thus print quality deteriorates.
A shock absorbent cushion design for transforming a body of displaceable material to react under pressure as if it was compressed thereby creating a cushioning effect. Controlled displacement is created by virtue of the design and the depth in a manner not to cause a rise before or after the nip point in the displacement of said material which would normally occur without the design of the present invention.
Generally, the present invention is a shock absorber and cushion for use directly or indirectly under virtually any type of printing plate or offset blanket in order to compensate for variations in thickness, height and centricity of the printing cylinder and printing plate during the printing process. The invention includes a sheet of elastomeric material sized to be placed around the printing plate cylinder, blanket cylinder, sleeve, or platum, said elastomeric sheet having predisposed displacement zones resulting from creating voids within the elastomeric material of predetermined thickness providing a path of least resistance for the displacement material for maintaining an even impression along the printing plate both across and around the printing plate cylinder.
The sheet of elastomeric material includes a predetermined geometric pattern that define the displacement zones which are preferably, although not limited to, circumferential in direction, i.e. linear raised protrusions that, in the preferred embodiment, extend in the direction of the printing path and that can be in parallel rows, spaced apart, circumferentially around the printing cylinder and in the direction of the printing drum rotation. The elastomeric sheet in accordance with the present invention has a plurality or an array of spaced-apart zone displacements of a predetermined geometrical cross-sectional shape and size, that may be evenly or randomly spaced apart and which are preferably in a parallel array in the direction of the rotation of the printing cylinder (substantially circumferential) relative to the printing cylinder. The spaced-apart displacement zones allow the elastomeric material to be radially displaced into the adjacent displacement zone to accommodate variations in thickness, height and centricity of both the printing plate and the print cylinder to which it is mounted.
In the preferred embodiment, the protrusions are linearly disposed. However, the linearity of the protrusions could vary and still provide the necessary radial displacement. The geometric pattern is designed to deliver varying amounts and levels of displacement or compression resistance thus controlling the impression required for fine graphics, while providing the resilience and modulus necessary to print large solids. Virtually any geometric orientation of protrusions may provide the necessary displacement zone. By way of example only, the geometric shapes shown in the accompanying drawings will provide the necessary displacement zone. The displacement zone is a combined product of the geometric cross-sectional area and shape itself and is greatly influenced by the distance placed between these protrusion elements as well as the durometer, resilience modulus and volume of the elastomeric material.
It is important that any geometric orientation of protrusions run (with or without break) in-press direction around the print cylinder. The press direction is described as the direction the printed material travels through the press. However, it is within the spirit of the invention to provide an orientation of protrusions which maintain the same cushioning and displacement characteristics, yet do not run in truly xe2x80x9cin-pressxe2x80x9d direction, but are situated in a substantially oblique relationship with respect to the direction of cylinder rotation.
In the preferred embodiment, the cushioning element is compromised of two layers: one is the base layer which consists of any stable layer of flexible material such as paper or cellophane, or any dimensionally stable layer of flexible material such as any metallic, polyester or vinyl material, or MYLAR(trademark). This material is used as a stabilizing base for the second layer which contains the geometric protrusion array all made of the same elastomeric material of a predetermined durometer whose resilience at normal operating temperatures will deform and fill the adjacent displacement areas under various amounts of stress. The cross-sectional shape of each protrusion strip may be a trapezoid, by way of example. The strips, spaced uniformly or randomly apart, are spaced apart at a predetermined distance and disposed across or around the cylinder width. Suitable elastomeric materials include, but are not limited to, polybutadience, polyisoprene, polychloroprene; and olefin copolymers such as styrene-butadiene copolymers, natural or synthetic rubbers (e.g. acrylonitrile-butadiene copolymer), ethylene-propylene copolymer, butyl rubber and foam rubber (e.g., isobutylene-isoprene copolymer). Elastomers which are thermoplastic are also suitable as the cushion layer and include, but are not limited to, styrene-diene-styrene triblock copolymers, such as polystyrene-polybutadiene-polystyrene (SBS), polystyrene-polyisoprene-polystyrene (SIS, or polystyrene-poly (ethylenebutylene)-polystyrene (SEBS); thermoplastic polyester and polyurethane elastomers; fluoroelastomers and thermoplastic polyolefin rubbers (polyolefin blends). Suitable elastomers also include chlorosulfonated polyethylene, polysulfide, polyalkylene oxides, polyphosphazenes, elastomeric polymers and copolymers of acrylates and methacrylates, and elastomeric copolymers of vinyl acetate and its partially hydrogenated derivatives.
In an alternate embodiment, the geometric protrusions themselves could be made of two or more layers of materials of different durometers, resilience or modulus to further efficiently control the resistance. The required resistance may vary and be altered to respond to the various print market such as corrugated, newsprint, poly/plastic and paper which may require different resistances. Being able to control these individual factors, a wide range of refinements for various cushioning requirements are possible. These protrusions from top to base formed from the elastomeric material should be greater than 5% of the total volume from floor to ceiling and should not exceed 95%. The most preferable ratio is between 10% and 50% volume of material to displacement void. The area adjacent the protrusion material mass shall be considered displacement void zones. The embodiment of this displaceable protrusion material is currently created by using photopolymer plate material from various manufacturers including but not limited to Dupont""s xe2x80x9cCyrel(copyright)xe2x80x9d, Polyfiberons"" xe2x80x9cEpic(copyright)xe2x80x9d and BASF""s xe2x80x9cNyloFlex(copyright)xe2x80x9d photopolymerizable, photocrosslinkable or both. The photopolymerizable layer compromises an elastomeric binder, at least one monomer and an initiator, where the initiator is preferably a photoinitiator having sensitivity to actinic radiation. Any photopolymerizable compositions which are suitable for the formation of flexographic printing plates can be used for the present invention. Examples of suitable compositions have been disclosed, for example, in Chen et al, U.S. Pat. No. 4,323,637, Gruetzmacher et al, U.S. Pat. No. 4,427,749 and Feinbert et al., U.S. Pat. No. 4,894,315.
The processes available to manufacture the cushion include laser engraving, mechanical engraving, molding, vulcanizing, embossing, extruding and other current technologies.
The cushion is mounted between the plate cylinder surface and the printing plate base. The cushion may be glued to the cylinder surface and to the printing plate surface. Alternatively, sticky tape may be used to attach the cushion with adhesive to the cylinder surface and also to the printing plate surface.
The cushion may be mounted so that the protrusions engage the bottom of the printing plate or conversely such that the protrusions engage the surface of the plate cylinder. In the preferred embodiment, the protrusions would engage the plate cylinder surface and be essentially inverted relative to the plate surface. Whether upside down or right-side up, depending on the point of view, it is important that the protrusion strips, in the preferred embodiment, be disposed to run parallel or in the direction of the circumferential drum rotation. The cushion material could also be used with any other printing plate mounting system that may include vacuum, clamps, sleeves, pins or other mechanical attachment.
The cushion described herein may be placed on a cylinder or sleeve sized to fit the outer diameter of the cylinder on which the plate is directly mounted. An alternative is to mount the printing plate on a second sleeve designed to fit over the initial sleeve or cylinder that the cushion layer is attached to.
The cushion, in accordance with the present invention, can be mounted onto the plate cylinder with adhesive, glue or double-sided adhesive tape. Thus, the cushion is directly against the printing plate, or indirectly, if you consider that glue or adhesive tape holds the cushion to the cylinder and to the printing plate.
In alternate embodiments, the support base and/or the protrusions could be added during the manufacturing process on the back of the printing plate so that it becomes part of the printing plate itself. In that case, the cushion and plate would be together as one single entity and then would be mounted by adhesive or other fastener onto the print cylinder. It is also in another embodiment possible that the plate cylinder surface itself could include permanently a particular cushion. And yet another possible alternate embodiment would be that the cushion, during the manufacture or finish, become a permanent part of the cylinder of sleeve.
It is an object of this invention to provide an improved cushion or shock absorber for use in a printing process to compensate for variations in thickness, height and centricity of the materials and equipment used in flexographic and other types of printing to enhance image quality and efficiency without increasing printing pressure which distorts the ultimate printed image.
It is another object of this invention to provide a cushion between a printing plate and printing drum that retains its resiliency without fatigue over extremely long printing runs without reducing image quality.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.